Chapter 3: Monitoring & Analysis
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usable IP addresses is a unique identifier that distinguishes one network node from another. Interconnected networks must agree on an IP addressing plan. IP addresses must be unique and cannot be used in different places on the Internet at the same time; otherwise, routers would not know how best to route packets to them.
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Internet
Server 10.1.1.2
PC
Server 10.1.1.2
Figure 3.3: Without unique IP addresses, unambiguous global routing is impossible. If the PC requests a web page from 10.1.1.2, which server will it reach?
IP addresses are allocated by a central numbering authority, which provides a consistent and coherent numbering method. This ensures that duplicate addresses are not used by different networks. The authority assigns large blocks of consecutive addresses to smaller authorities, who in turn assign smaller consecutive blocks within these ranges to other authorities, or to their customers. These groups of addresses are called sub-networks, or subnets for short. Large subnets can be further subdivided into smaller subnets. A group of related addresses is referred to as an address space.
Subnets By applying a subnet mask (also called a network mask, or simply netmask) to an IP address, you can logically define both a host and the network to which it belongs. Traditionally, subnet masks have been expressed using dotted decimal form, much like an IP address. For example, 255.255.255.0 is one common netmask. You will find this notation used when configuring network interfaces, creating routes, etc. However, subnet masks are more succinctly expressed using CIDR notation, which simply enumerates the number of bits in the mask after a forward slash (/). Thus, 255.255.255.0 can be simplified as